1. Field of the Invention
The present invention relates to a magnetic recording medium with reduced noise at high density recording and high Output which are required in, for example, a small size video tape with high picture quality. In particular, the present invention provides a magnetic recording medium, such as a magnetic tape, which has a magnetic layer and a substrate, a backcoating layer having a total thickness of not greater than 14 .mu.m used in a wound form, such that the noise does not increase after long term storage.
2. Description of the Related Art
In general, a magnetic recording medium comprises a substrate made of, for example, a polyester film and a magnetic layer formed on a surface of the substrate by applying a magnetic paint which comprises a magnetic powder, a binder, inorganic powder, a lubricant and the like dispersed in an organic solvent and drying it. In particular, in case of a video tape, a backcoating layer comprising a binder, inorganic powder, a lubricant and the like is formed on a back of the substrate to improve running properties.
To prevent abrasion during contact to a magnetic head or guide rollers, it is proposed to add inorganic powder having a Mohs' hardness of 5 or higher to both the magnetic layer and the backcoating layer.
The addition of the inorganic powder having Mohs' hardness of 5 or higher to the backcoating layer is disclosed in Japanese Patent Kokai Publication Nos. 112/1987, 38525/1987, 38526/1987, 38527/1987 and 38528/1987 and the addition of the inorganic powder having Mohs' hardness of 5 or higher to the magnetic layer is disclosed in Japanese Patent Publication Nos. 1857/1972, 15003/1973, 39402/1974, 28642/1977, 49961/1977 and 15771/1980.
To achieve a high density recording, it is necessary to shorten the recording wavelength. Therefore, the magnetic powder having a particle size smaller than the recording wavelength is used.
To obtain high output, many proposals have been made to improve magnetic characteristics. For example, Japanese Patent Publication No. 37761/1986 discloses the use of ferromagnetic metal powder having high saturation magnetization to achieve high output.
In addition, to decrease the noise, Japanese Patent Publication Nos. 17404/1977 and 12688/1985 disclose finishing of the surface of the magnetic layer by a super calendering treatment to smoothen the surface.
However, while the magnetic recording medium, which comprises the ferromagnetic metal powder as a magnetic recording element and is produced by surface finishing, has a low original noise, the noise increases after storage.
This problem is remarkable when the magnetic powder has a small particle size and a metal magnetic powder which is softer than iron oxide base magnetic powder is used, and it is found that the noise against a signal has a frequency corresponding to recording wavelengths not longer than about 0.8 .mu.m. This problem is found in almost all prototype magnetic recording media, while the prototype magnetic recording media having no backcoating layer keeps the original low noise.
Therefore, the present inventors thinking that this problem might be caused by the backcoating layer have made extensive study to find that surface roughness of the backcoating layer could be transferred to the surface of the magnetic layer when the magnetic recording medium is wound and the transferred roughness might cause noise.
It was found that the increase of noise was remarkable in a winding type magnetic recording medium having a total thickness of 14 .mu.m or less of the substrate, the magnetic layer and the backcoating layer. This is because such a thin magnetic recording medium is wound in a larger number of turns than a thick magnetic recording medium having a total thickness of about 20 .mu.m in a unit winding diameter, and a near hub, winding force is larger.
Japanese Patent Kokal Publication No. 32122/1985 describes the similar phenomena and proposes to prevent flaws of tape guides by using a combination of inorganic powder having Mohs' hardness of 5 or higher and a particle size of not larger than 0.2 .mu.m and a soft inorganic powder in an amount larger than the former inorganic powder.
Japanese Patent Kokai Publication No. 255584/1988 recognizes the decrease of S/N and C/N of the magnetic recording medium by the transfer of the surface roughness of the backcoating layer to the surface of the magnetic layer and describes that an amount of a solvent remained in the magnetic layer may relate to this problem.
However, the above patent literature relates to cobalt-containing iron oxide magnetic powder and a VHS type magnetic recording medium having the total thickness of about 20 .mu.m of the substrate, the magnetic layer and the backcoating layer. Since in the VHS system, the shortest recording wavelength of the white level is only 1.3 .mu.m, the above patent literature does not suggest the decrease of noise in the magnetic recording medium which comprises the ferromagnetic metal powder as the magnetic recording element and uses a center recording wavelength of 0.8 .mu.m.